Think of EMG's as an extension of the physical examination. When someone has the clinical symptoms and physical findings for carpal tunnel, you may use it to tell you how severe the carpal tunnel is, as well as, how the disorder progresses over time, and, if the diagnosis is in doubt, prove or disprove it.
EMG's are actually composed of two parts: the nerve conduction studies (NCS) as well as the electromyogram (EMG).
The NCS part involves studying the nerves themselves. Electricity is sent from one part of the nerve to a different part of the nerve. The time it takes to travel from point A to point B is divided over the measured distance between the two points (mm/msec) and a conduction velocity calculated.
Also, the amount of nerve fibers transmitting can be measured by the size (amplitude), of the electrical pulse received. Thus, the health of the nerve can be measured objectively.
The EMG part involves studying the muscles. A pin is inserted into different muscles and the electrical activity in the muscles is picked up.
When there is muscle damage, as from a muscle disorder (myopathy), the electrical signals are altered. The normal triphasic wave (similar in appearance to the QRS wave of an EKG), becomes shorter and more narrow.
Also, when there is damage to the nerves innervating the muscle (ie. radiculopathies), the normal triphasic wave is again altered. Depending on how long the damage has occurred, different changes can be seen.
These include increased wave complexity, wave size, and number of waves.
Moreover, when either nerve or muscle are damaged, there will be spontaneous electrical activity (fibrillations), not normally seen.
Common reasons that physicians order EMG's include numbness, paresthesias (pins/needles or other strange sensations), weakness, and pain that are otherwise unexplained and/or need further evaluation.
Examples include: carpal tunnel syndrome (median neuropathy at the wrist), cubital tunnel syndrome (ulnar neuropathy at the elbow), cervical, thoracic, or lumbar radiculopathies, peripheral neuropathy, myopathies, cranial nerve palsies, brachial plexopathies, neuromuscular disorders (ALS, myotonic conditions, etc.)
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